#! /usr/bin/env python
#
def r8_uniform_01 ( seed ):

#*****************************************************************************80
#
## R8_UNIFORM_01 returns a unit pseudorandom R8.
#
#  Discussion:
#
#    This routine implements the recursion
#
#      seed = 16807 * seed mod ( 2^31 - 1 )
#      r8_uniform_01 = seed / ( 2^31 - 1 )
#
#    The integer arithmetic never requires more than 32 bits,
#    including a sign bit.
#
#    If the initial seed is 12345, then the first three computations are
#
#      Input     Output      R8_UNIFORM_01
#      SEED      SEED
#
#         12345   207482415  0.096616
#     207482415  1790989824  0.833995
#    1790989824  2035175616  0.947702
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license. 
#
#  Modified:
#
#    17 March 2013
#
#  Author:
#
#    John Burkardt
#
#  Reference:
#
#    Paul Bratley, Bennett Fox, Linus Schrage,
#    A Guide to Simulation,
#    Second Edition,
#    Springer, 1987,
#    ISBN: 0387964673,
#    LC: QA76.9.C65.B73.
#
#    Bennett Fox,
#    Algorithm 647:
#    Implementation and Relative Efficiency of Quasirandom
#    Sequence Generators,
#    ACM Transactions on Mathematical Software,
#    Volume 12, Number 4, December 1986, pages 362-376.
#
#    Pierre L'Ecuyer,
#    Random Number Generation,
#    in Handbook of Simulation,
#    edited by Jerry Banks,
#    Wiley, 1998,
#    ISBN: 0471134031,
#    LC: T57.62.H37.
#
#    Peter Lewis, Allen Goodman, James Miller,
#    A Pseudo-Random Number Generator for the System/360,
#    IBM Systems Journal,
#    Volume 8, Number 2, 1969, pages 136-143.
#
#  Parameters:
#
#    Input, integer SEED, the integer "seed" used to generate
#    the output random number.  SEED should not be 0.
#
#    Output, real R, a random value between 0 and 1.
#
#    Output, integer SEED, the updated seed.  This would
#    normally be used as the input seed on the next call.
#
  from sys import exit

  i4_huge = 2147483647

  seed = int ( seed )

  seed = ( seed % i4_huge )

  if ( seed < 0 ):
    seed = seed + i4_huge

  if ( seed == 0 ):
    print ( '' )
    print ( 'R8_UNIFORM_01 - Fatal error!' )
    print ( '  Input SEED = 0!' )
    exit ( 'R8_UNIFORM_01 - Fatal error!' )

  k = ( seed // 127773 )

  seed = 16807 * ( seed - k * 127773 ) - k * 2836

  if ( seed < 0 ):
    seed = seed + i4_huge

  r = seed * 4.656612875E-10

  return r, seed

def r8_uniform_01_test ( ):

#*****************************************************************************80
#
## R8_UNIFORM_01_TEST tests R8_UNIFORM_01.
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license.
#
#  Modified:
#
#    26 July 2014
#
#  Author:
#
#    John Burkardt
#
  import platform

  print ( '' )
  print ( 'R8_UNIFORM_01_TEST' )
  print ( '  Python version: %s' % ( platform.python_version ( ) ) )
  print ( '  R8_UNIFORM_01 produces a sequence of random values.' )

  seed = 123456789

  print ( '' )
  print ( '  Using random seed %d' % ( seed ) )

  print ( '' )
  print ( '  SEED  R8_UNIFORM_01(SEED)' )
  print ( '' )
  for i in range ( 0, 10 ):
    seed_old = seed
    x, seed = r8_uniform_01 ( seed )
    print ( '  %12d  %14f' % ( seed, x ) )

  print ( '' )
  print ( '  Verify that the sequence can be restarted.' )
  print ( '  Set the seed back to its original value, and see that' )
  print ( '  we generate the same sequence.' )

  seed = 123456789
  print ( '' )
  print ( '  SEED  R8_UNIFORM_01(SEED)' )
  print ( '' )

  for i in range ( 0, 10 ):
    seed_old = seed
    x, seed = r8_uniform_01 ( seed )
    print ( '  %12d  %14f' % ( seed, x ) )
#
#  Terminate.
#
  print ( '' )
  print ( 'R8_UNIFORM_01_TEST' )
  print ( '  Normal end of execution.' )
  return

if ( __name__ == '__main__' ):
  from timestamp import timestamp
  timestamp ( )
  r8_uniform_01_test ( )
  timestamp ( )